Reliable estimates of crocodylian population size are desirable for both understanding theecology and natural history of species and developing sound conservation and management plans. How-ever, choosing appropriate methods to estimate population numbers can be difficult due to the paucity ofcomprehensive analyses regarding their effectiveness, robustness, and applicability. We estimated theAmerican crocodile population size in the southern tip of Coiba Island, Panama, using both spotlight sur-veys (Messel?s and King?s visible fraction estimations) and mark?recapture (POPAN formulation?super-population) methods. We assessed and compared the outcomes of these methods with the overall capturerecord for the study area from 2009 to 2013, evaluating their applicability, accuracy, strengths, and limita-tions. Using historical and current capture data, we defined a minimum population size of~112 non-hatchling animals in our study area, which was larger than both Messel?s (19.00 7.50 individuals) andKing?s (25.71 7.25 individuals) population size estimates, revealing that these latter approaches clearlyunderestimate population numbers. We estimated a total population size that range between 147 and 257individuals based on POPAN formulation grouping the data by sex and age groups as the most plausiblepopulation size of the American crocodile population in this area at the time. We analyzed and discussedsources of bias in population size estimations for all methods used in the present study, providing recom-mendations to minimize errors and improve estimations. Finally, we analyzed and compared populationecology attributes obtained in our study with what have been reported in other insular and coastal areasacross the American crocodile range, increasing knowledge about the ecology of the species (18) (PDF) How to estimate population size in crocodylians? Population ecology of American crocodiles in Coiba Island as study case. Available from: https://www.researchgate.net/publication/328433568_How_to_estimate_population_size_in_crocodylians_Population_ecology_of_American_crocodiles_in_Coiba_Island_as_study_case [accessed Dec 11 2018].